The present invention relates to the transmission of data in a telecommunications network. More particularly, the present invention relates to detecting and locating segments responsible for corrupting or degrading telecommunications data along a telecommunications connection, for example, an asynchronous transfer method adaptation layer number two (AAL2) connection, traversing a number of switching points.
Asynchronous transfer mode (ATM) is a standard protocol for transmitting telecommunications data across a telecommunications network. It is based on the transmission of data in fixed size data packets, known as ATM cells, from a source node in the telecommunications network to a destination node via any number of intermediate switching nodes. Each ATM cell has a 48 octet payload portion and a five octet header portion. ATM is well known in the art.
ATM is frequently used for transporting low bit rate data in telecommunications networks, for example, low bit rate voice data. However, standard ATM does not efficiently transport low bit rate data. More specifically, standard ATM tends to add a significant time delay to the transmission of low bit rate data. Unfortunately, low bit rate data, such as low bit rate voice data, is highly sensitive to transmission delays. Consequently, several different ATM adaptation layers (AALs) have been developed to improve the efficiency ATM with respect to the transportation of low bit rate data, as is well known in the art.
AAL2 is one of the AALs which have been developed to improve the efficiency of ATM with respect to the transmission of low bit rate data. AAL2 is also well known in the art, and it is more thoroughly described in the B-ISDN ATM Adaptation layer Type 2 Specification, ITU Recommendation I.363.2 (herein referred to as "the AAL2 specification"). AAL2 makes ATM a more efficient vehicle for transporting low bit rate data by inserting low bit rate data from a number of different sources into AAL2 data packets, and then multiplexing the AAL2 data packets from the various sources onto a single ATM connection.
In accordance with the AAL2 specification, the standard format of an AAL2 data packet is similar to that of a standard ATM cell. For example, an AAL2 data packet also has a header portion and a payload portion. However, the header portion of an AAL2 data packet is 3 octets in length and the payload of an AAL2 data packet can vary from 1 octet to 64 octets.
The header portion of an AAL2 data packet more specifically comprises an 8 bit connection identifier (CID) field, a 6 bit length indicator (LI) field, a 5 bit user-to-user information (UUI) field, and a 5 bit header error control (HEC) code. The CID field defines the AAL2 channel to which the corresponding AAL2 packet belongs. According to the AAL2 specification, the AAL2 packet may be associated with one of 248 different AAL2 channels. Therefore, AAL2 data packets from as many as 248 different data sources can be multiplexed onto a single ATM connection. The LI field, as the name suggests, defines the length of the payload portion of the AAL2 data packet (e.g., the number of octets in the payload). The HEC code is specifically used for detecting errors in the header portion of the AAL2 packet.
There are 5 bits in the UUI field. Therefore, 32 different binary code combinations are possible. Two of these binary code combinations are specifically reserved for identifying the AAL2 data packet as an operation and maintenance (OAM) data packet. The first of these binary code combinations specifically identifies the AAL2 packet as an end-to-end packet. An end-to-end packet is always transmitted from a first node to a second node, transparent to any intermediate nodes located along the connection between the first and the second nodes. The second binary code combination identifies the AAL2 packet as a segment packet. A segment packet is transmitted from a first node to a second node adjacent to the first node, then re-transmitted back to the first node.
Standard ATM, in accordance with ITU-T Recommendation I.610, B-ISDN Operation and Maintenance Principles and Functions, herein referred to as the ATM OAM specification, provides an OAM performance monitoring function. The purpose of this performance monitoring function is to check the overall quality of the ATM connection. In accordance with the ATM OAM specification, the overall quality of the ATM connection is measured in terms of the number of ATM cells lost and the number of ATM cell payload bit errors.
In practice, the ATM OAM performance monitoring function is very complex. For example, degradation is typically identified by a connection endpoint such as a destination node. Consequently, the destination node is only aware that a degrading condition exists somewhere between the source node and the destination node. However, the precise location of the segment responsible for the degrading condition is not determined. Accordingly, the network manager must check each and every segment individually and on a regular basis, whether or not there is any indication that a performance degradation exists. This, in turn, results in an unnecessary expenditure of time and network resources (e.g., bandwidth). Therefore, a more simplified and efficient procedure is needed to detect and precisely locate segments causing data degradation along a telecommunications connection within a telecommunications network, and to do so on an "as needed" basis rather than on a periodic basis.